In an internal combustion engine, it is known to deliver fuel into the cylinders of the engine by means of a fuel injector. One type of fuel injector that permits precise metering of fuel delivery is a so-called ‘piezoelectric injector’. Typically, a piezoelectric injector includes a piezoelectric actuator that is operable to control movement, directly or indirectly, of a valve needle between injecting and non-injecting states. The valve needle is engageable with a valve needle seating to control fuel delivery through one or more outlet openings in the nozzle of the injector. A hydraulic amplifier may be situated between the actuator and the needle such that axial movement of the actuator causes an amplified axial movement of the needle. An example of a piezoelectric injector of the aforementioned type is described in EP 0995901.
The piezoelectric actuator comprises a stack of piezoelectric elements which, as a whole, are electrically equivalent to a capacitor having a particular capacitance. Changing the voltage applied across the piezoelectric stack alters the amount of electrical charge stored by the stack (also known as its “energisation level”) and, therefore, the axial length of the piezoelectric stack. By varying the length of the stack and, thus, the position of the valve needle relative to the seating, the amount of fuel that is passed through the fuel injector can be controlled. In this way, piezoelectric fuel injectors offer the ability to meter precisely a small amount of fuel. A known piezoelectrically operated fuel injector of the aforementioned type is described in our co-pending European patent application EP 1174615.
The amount of charge applied to and removed from the piezoelectric actuator can be controlled in one of two ways. In a charge control method, a current is driven into or out of the piezoelectric actuator for a period of time so as to add or remove, respectively, a demanded charge to or from the stack, respectively. Alternatively, in a voltage control method a current is driven into or out of the piezoelectric actuator until the voltage across the piezoelectric actuator reaches a demanded (predetermined) differential voltage level. In either case, the voltage across the piezoelectric actuator changes as the level of charge on the piezoelectric actuator varies (and vice versa).
Typically, an engine has more than one fuel injector, which may be grouped together in banks of one or more injectors. As described in EP 1400676, each bank of injectors may have its own drive circuit for controlling operation of the injectors. The circuitry includes a power supply, such as a transformer, which steps-up the voltage generated by a power source (e.g. from 12 volts to a higher voltage); and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors, which are used to power the charging and discharging of the piezoelectric fuel injectors for each injection event. Drive circuits have also been developed, as described in WO 2005/028836A1, which do not require a dedicated power supply, such as a transformer.
In order to initiate an injection of fuel, the drive circuit may be used to cause the differential voltage across the actuator terminals to transition from a high level at which no fuel delivery occurs to a relatively low level at which fuel delivery occurs. An injector responsive to this “drive waveform” is referred to as a “de-energise to inject” injector. Hence, when such de-energise to inject injectors are in their non-injecting state, the voltage across the piezoelectric actuator of the injector is relatively high; whereas in an injecting state the voltage across the actuator is relatively low. Since each fuel injection event is generally relatively rapid, the piezoelectric actuator may be fully energised for approximately 95% of the operating life span.
It has been recognised, however, that the existence of such a high voltage across the piezoelectric actuator for a relatively long portion of the operating cycle of the actuator may cause the degradation (“aging”) of the piezoelectric stack, leading to a change in its mechanical and/or electrical properties and, thus, adversely affecting the life span (durability) and performance of the injector. These problems may be attributable, in part, to the higher stress levels exerted on the piezoelectric actuator at the higher differential voltage levels in a non-injecting state. It is also suspected that a high voltage across the terminals of the actuator may encourage the permeation of ionic species into the actuator though its protective actuator encapsulation. In any event, any resultant inaccuracies in fuel volume delivery will have a detrimental effect on combustion efficiency and lead to worse fuel economy and increased exhaust emissions.
It would, therefore, be desirable to provide a piezoelectric actuator-controlled fuel injector that is not subjected to such high differential voltages for such a high proportion of its operating cycle, so as to increase the operational life of the injector and beneficially to maintain fuel injection quantity accuracy.
It would be further advantageous to provide a method of operating a piezoelectric actuator-controlled fuel injector in such a way as to increase the longevity of the injector, and enhance or maintain its ability to deliver predictable and accurate fuel injection quantities.
Thus, the invention relates to a method for operating a piezoelectric fuel injector so as to overcome or at least alleviate at least one of the above-mentioned problems.